Devensian Ice Sheet Research Articles

Glacitectonic evidence of ice sheet interaction and retreat across the western part of Dogger Bank (North Sea) during the Last Glaciation

High-resolution 2D seismic data from the western side of Dogger Bank (North Sea) has revealed that the glacigenic sediments of the Dogger Bank Formation record a complex history of sedimentation and penecontemporaneous, large-scale, ice-marginal to proglacial glacitectonism. The resulting complex assemblage of glacial landforms and sediments record the interplay between two separate ice masses revealing that Late Devensian ice sheet dynamics across Dogger Bank were far more complex than previously thought, involving the North Sea lobe of the British and Irish Ice Sheet, advancing from the west, interacting with the Dogger Bank lobe which expanded from the north. The active northward retreat of the Dogger Bank lobe resulted in the development of a complex assemblage of arcuate thrust-block moraines (≤15 km wide, >30 km long) composed of highly folded and thrust sediments, separated by sedimentary basins and meltwater channels filled by outwash. The impact of the North Sea lobe was restricted to the western margin of Dogger Bank and led to deep-seated (100–150 m thick) glacitectonism in response to ice-push from the west. During the earlier expansion of the North Sea lobe, this thrust and fold complex initially occupied a frontal marginal position changing to a more lateral ice-marginal position as the ice sheet continued to expand to the south. The complex structural relationships between the two glacitectonic complexes indicates that these ice masses interacted along the western side of Dogger Bank, with the inundation of this area by ice probably occurring during the last glaciation when the ice sheets attained their maximum extents.

The susceptibility of glacigenic deposits to liquefaction under seismic loading conditions: a case study relating to nuclear site characterization in West Cumbria

Previous research has established a chronostratigraphy of glacial/deglacial events together with a lithostratigraphical framework for the Quaternary deposits of the Sellafield area in West Cumbria. The glacial record is dominated by sediments and landforms laid down during the deglaciation of the Devensian ice sheet. The Quaternary deposits are characterized by complex sequences resulting from oscillating ice sheet margins, changing glacial lake and outwash environments, relative sea level changes and glacio-tectonic deformation. The resulting deposits comprise sequences which possess a wide range of grain size, shape and sorting; some are normally consolidated and have very low to low in situ density. An understanding of the geotechnical behaviour of these deposits is critical during geological site characterization studies for the design of new buildings and infrastructure on existing and proposed nuclear sites. The geotechnical engineering behaviour of a sequence of glacial deposits on the Sellafield nuclear site is evaluated together with their susceptibility to potential liquefaction under certain seismic load conditions. The evaluation identified that layers of very loose to loose deposits within the ice sheet marginal landsystem are particularly susceptible to liquefaction under specified seismic loading conditions that are required to be modelled by the UK Office for Nuclear Regulation (ONR).

The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution

A region of high heads within the Borrowdale Volcanic Group (BVG; a fractured crystalline rock) beneath the coastal plain of West Cumbria, England (UK), is identified as a possible relic left over by the Late Devensian ice sheet. It was found during investigations in the 1990s. Contemporary modelling work failed to produce a satisfactory explanation of the high heads compatible with the ‘cold recharge’ isotopic signature of the groundwater. This study has reassessed the original hydraulic testing results. By plotting density-adjusted heads versus their depth below the water table in the immediate vicinity of the borehole in which they were measured, a depth profile resembling a ‘wave’ was revealed with a peak value located at 1,100 m depth. The possibility that this wave represents relic heads from the last major ice sheet has been assessed using one-dimensional mathematical analysis based on a poroelastic approach. It is found that a wet-based ice sheet above the West Cumbrian coast was probably thick enough and sufficiently long-lasting to leave such relic heads providing that the hydraulic diffusivity of the BVG is in the order of 10−6 m s−1. Initial assessment 20 years ago of the long-interval slug tests suggested that such low values are not likely. More recent interpretation argues for such low values of hydraulic diffusivity. It is concluded that ice sheet recharge is the most likely cause of the raised heads, that the BVG contains significant patches of very low conductivity rock, and that long-interval single-hole tests should be avoided in fractured crystalline rock.

Northern sector of the Uummannaq Ice Stream System, West Greenland – Deglacial behaviour and Holocene climatic variability

Glacial bedform patterns and sediments deposited by the temperate and polythermal Late Devensian ice sheet in north-central Ireland record changes in the processes, location, and magnitude of subglacial meltwater throughout the last full glacial cycle (∼21–14 14C kyear BP). Meltwater characteristics are related directly to basal ice thermal regime and ice dynamics, including ice velocity and shifts in the location of ice centres. Therefore, reconstructed meltwater characteristics may provide insight into wider controls on dynamic ice behaviour. A range of meltwater-related features are present across north-central Ireland. These include tunnel valleys, drumlin leeside sequences, eskers, and boulder lags. Other bedforms including Rogen moraines were modified by meltwater activity along ice streams. Meltwater was stored subglacially in two contrasting regions located beneath or near ice centres in north-central Ireland. (1) The Lough Erne Basin is developed in a lowland depression occupied partly by subglacial Rogen moraine ridges which were formed around the time of the last glacial maximum. Meltwater was stored between Rogen ridge crests and released by hydraulic jacking associated with drumlinisation (∼16.6 14C kyear BP) and ice streaming (∼13.8 14C kyear BP). (2) The Lough Neagh Basin occupies a similar lowland depression and was the location of an ice sheet centre throughout the last glacial cycle. No bedforms are present beneath or immediately surrounding Lough Neagh. A larger, more continuous meltwater lake existed in the Lough Neagh depression, probably sealed by a region of cold-based ice outside lake margins. Water escaped through regional-scale tunnel valleys, particularly the Poyntzpass channel which was active during the Carlingford ice readvance (Killard Stadial, correlated with Heinrich event 1 at ∼14.5 14C kyear BP). Overall, reconstructed subglacial lake characteristics and drainage mechanisms are related closely to basal ice thermal regime and substrate relief (controlling lake geometry), and provide insight into controls on overall ice sheet dynamics.

Lateglacial surface exposure dating in the Monadhliath Mountains, Central Highlands, Scotland

We constrain the deglaciation history of the Monadhliath Mountains since the Last Glacial Maximum using glacial geomorphology and surface exposure dating. Collectively these data clarify the glacial history of the Monadhliath Mountains at the close of the Devensian cold period. 10Be exposure ages indicate deglaciation of the Last Devensian ice sheet at 15.1 ± 1.4 ka at ca 640 m OD. The exposure ages are consistent with basal radiocarbon ages from nearby Loch Etteridge, 5 km to the south-east (15.3 ± 0.2 kcal BP11ka is used for ages in this study, kcal BP for calibrated ages and 14C kyrs BP for uncalibrated ages 1.). Boulders from moraines in three Monadhliath corries yielded exposure ages between 11.8 ka and 9.8 ka (470–600 m OD), suggesting that a Lateglacial advance occurred during the Younger Dryas stadial. The limited extent of these Younger Dryas glaciers in the Monadhliath Mountains results from the combined effect of a drier climate experienced in the eastern part of the Central Highlands ice cap and topographical factors. Our reconstruction largely supports the deglaciation model of Golledge et al. (2008) and confirms that a SW to NE precipitation gradient dominated Scotland during the Younger Dryas.

In situ cosmogenic exposure ages from the Isle of Skye, northwest Scotland: implications for the timing of deglaciation and readvance from 15 to 11 ka

AbstractWe present 10 in situ cosmogenic exposure ages from two moraines on the Isle of Skye. The Strollamus medial moraine was deposited during deglaciation of the Devensian ice sheet and yields a mean exposure age from five samples of 14.3 ± 0.9 ka. The moraine age indicates that a significant ice mass existed on Skye at the time of a regional readvance recorded in Wester Ross, northwest Scotland. Taken at face value the ages suggest that deglaciation did not occur until well into Greenland Interstade 1. The Slapin moraine represents the local limit of the Loch Lomond Readvance (LLR) and yields a mean exposure age from five samples of 11.5 ± 0.7 ka, which is consistent with deposition relating to the LLR. These ages suggest that the maximum extent may have been reached late in the stadial and that some glaciers may have remained active until after the climatic amelioration that marks its end. This scenario is considered unlikely given the nature of the climate during this period, which leads us to call for a locally calibrated production rate. Copyright © 2011 John Wiley & Sons, Ltd.

Discussion of ‘Late Devensian palaeoenvironmental changes in the sea area adjacent to Islay, SW Scotland: implications for the deglacial history of the island’ by J.D. Peacock, Scottish Journal of Geology , 44, 183–190

A.G. Dawson writes : The paper by Peacock (2008) represents a valuable contribution to our understanding of the Late Quaternary history of Islay in particular and the SW Scottish Hebrides as a whole. As part of the conclusions based on examination and dating of three cores (two from Loch Indaal and one from between SE Islay and SE Jura) Peacock argues that his interpretation is difficult to reconcile with the view presented by this author that the limit of the Late Devensian ice sheet lay SW–NE across Islay and is represented, in part, by the Central Islay moraine (Dawson 1982). The latter view is not a new one. For example, Sissons (1981) argued on several grounds (mostly on the basis of raised shoreline evidence) that the last (Late Devensian) Scottish ice sheet may have terminated amidst the Inner Hebridean islands. Of course, conventional wisdom would suggest that the limit of the last (Late Devensian) ice sheet lay far to the west of the Inner Hebrides, so why should an idea exist that this ice limit may have been located closer to the Scottish mainland? The initial discussion of this topic was presented by Dawson (1982) with new data described in several later papers (Benn & Dawson 1987; Dawson et al . 1997; Dawson & Dawson 2001). The interpretation of the regional deglacial history is based on geomorphological and stratigraphical evidence not mentioned in Peacock (2008). …

A palaeo-ice stream of the British Ice Sheet in eastern Scotland

Terrestrial and marine subglacial landforms in eastern Scotland are used to evaluate previously unsubstantiated notions of ice streaming within the British Ice Sheet (BIS) in this area during the last glacial cycle. Employing both regional and local-scale data sets, we describe onshore landform-sediment assemblages, offshore geomorphology and stratigraphy, and reconstructed palaeo-ice flow patterns. The results and their glaciological significance are discussed in the context of stratigraphical and geomorphological frameworks established by earlier workers, and are compared with modelled reconstructions for the BIS in this area. We conclude that the Main Late Devensian ice sheet in eastern Scotland hosted a zone of fast-flowing ice at least 100 km long and 45 km wide, akin to a contemporary ice stream. This sector / the Strathmore Ice Stream / flowed through a combination of basal sliding on meltwater-lubricated rigid beds and by deforming unconsolidated basal substrates

Landslide geomorphology of Cayton Bay, North Yorkshire, UK

SUMMARY The geomorphology of a large coastal landslide complex at Cayton Bay, North Yorkshire, is described. The area inland of the complex is occupied by a main road and a number of properties, so knowledge of current and possible future landslide behaviour is important for planning and risk assessment. Instability at the site is associated with a number of faults that bring argillaceous Upper Jurassic rocks to sea level. These soft rocks are overlain by more resistant sandstones. The sequence is capped by a thick and variable succession of glacial sediments, comprising tills with interbedded sand and gravel lenses that were deposited during the Dimlington Stadial of the Late Devensian. In connection with a study aimed at understanding cliff instability and recession, coastal erosion and the risk to people and assets at Cayton Bay, detailed geomorphological field mapping was conducted. This identified two major landslide systems, which include a periodically active mudslide complex at Cayton Cliff, recognized by shallow scarps and benches with occasional back-tilted blocks, and an area of dormant, deep-seated landslides at Tenants’ Cliffs, which includes a number of graben and horst structures. The origins of the landslides are unclear, but probably involved a variety of processes that led to a reduction in material shear strength or increases in pore water pressures. The original failure probably occurred in the middle Holocene, around 3000 years ago, when sea level reached its approximate current position. This would have initiated erosion of Jurassic argillaceous rocks (Oxford Clay) now exposed at sea level on the coast, and probable deep-seated failure of the overlying, more competent strata (Lower Calcareous Grit). Alternatively, it is possible that retreat of the Late Devensian ice sheet triggered instability, and that the landslide complex is a paraglacial landform produced during deglaciation, around 13 000 years BP. However sea levels were then lower so erosion of the Oxford Clay is unlikely to have been a factor in its development. Contemporary instability is likely to be due to the combined effects of coastal erosion and high groundwater levels, both of which are predicted to increase in the future due to the impact of climate change, implying an increased risk to coastal assets and a need to manage and mitigate such risks.

Allochthonous and indigenous palynomorphs from the Devensian of the Warham Borehole, Stiffkey, north Norfolk, England; evidence for sediment provenance

SUMMARY Late Devensian glaciolacustrine sediments proved by the Warham Borehole, near Stiffkey, north Norfolk, have yielded abundant and diverse recycled palynomorphs of Carboniferous, Permian, Triassic, Jurassic, Early Cretaceous and Palaeogene age. Indigenous Quaternary palynomorphs are a relatively minor component of the palynofloras; Carboniferous and Jurassic palynomorphs are the most commonly occurring forms. Middle Jurassic spores and pollen and Toarcian (Lower Jurassic) and Kimmeridgian (Upper Jurassic) dinoflagellate cysts are prominent, and the latter may indicate derivation from particularly resistant Toarcian and Kimmeridgian lithologies. Surprisingly, few grains were derived from the Chalk Group, which crops out locally. The recycled palynomorphs are interpreted as having been derived from local glacial deposits and then incorporated into the glaciolacustrine sediments of proglacial Lake Stiffkey, at the margin of the Late Devensian ice sheet. The lowest sample studied was the most productive palynologically. It was probably derived from nearby Lowestoft Till, an interpretation that agrees with the sedimentological evidence. The allochthonous palynomorphs from samples collected higher in the glaciolacustrine succession were probably derived from glaciogenic sediments deposited at the margin of the Late Devensian ice sheet. Based on the palynofloras, the Late Devensian ice flowed south from Scotland across the northern Pennines and NE England, down the western flank of the North Sea Basin to north Norfolk. The terrestrial nature of the indigenous Quaternary palynomorphs recovered is consistent with an ice-marginal and glaciolacustrine depositional setting.

Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland

The Cairngorm Mountain area of Scotland is a classic example of a landscape of selective linear glacial erosion, with sharp contrasts in the intensity of glacial erosion between the deeply incised troughs and valleys and the undulating high plateau. This article examines the Quaternary development of Glen Avon, a 200 m deep glacial trough set within the high plateau of the mountains. Evidence concerning the aggregate basal thermal regimes of the topographically controlled ice streams that formerly developed in this area is reconstructed from the geomorphological record, including bedforms indicative of wet‐based, sliding ice and of dry‐based ice frozen to its bed. This mapping indicates that basal sliding was not confined exclusively to the troughs but extended towards valley heads and on to parts of the plateau adjacent to troughs. The extent of basal sliding appears to have been greatest beneath pre‐Late Devensian ice sheets. Basal ice temperatures are modelled under steady‐state conditions for the last ice sheet at c. 18 ka BP. Basal thermal regimes are predicted using a reconstruction of the preglacial relief and for the current topography of the area. Convergent flow of ice through the preglacial valley system appears to have been sufficient to induce basal melting and therefore to initiate valley deepening. This effect is enhanced when the model is run across the present topography. Comparison of results of the geomorphological mapping and the modelling reveals significant differences between the actual and predicted extent of basal sliding outside the main ice stream. The overall conclusion is that many ice streams in mountainous terrain are inherited from the locations of preglacial valleys, which serve to accelerate ice flow and promote frictional heating beneath ice sheets.

Bedform patterns, subglacial meltwater events, and Late Devensian ice sheet dynamics in north-central Ireland

Glacial bedform patterns and sediments deposited by the temperate and polythermal Late Devensian ice sheet in north-central Ireland record changes in the processes, location, and magnitude of subglacial meltwater throughout the last full glacial cycle (∼21–14 14C kyear BP). Meltwater characteristics are related directly to basal ice thermal regime and ice dynamics, including ice velocity and shifts in the location of ice centres. Therefore, reconstructed meltwater characteristics may provide insight into wider controls on dynamic ice behaviour. A range of meltwater-related features are present across north-central Ireland. These include tunnel valleys, drumlin leeside sequences, eskers, and boulder lags. Other bedforms including Rogen moraines were modified by meltwater activity along ice streams. Meltwater was stored subglacially in two contrasting regions located beneath or near ice centres in north-central Ireland. (1) The Lough Erne Basin is developed in a lowland depression occupied partly by subglacial Rogen moraine ridges which were formed around the time of the last glacial maximum. Meltwater was stored between Rogen ridge crests and released by hydraulic jacking associated with drumlinisation (∼16.6 14C kyear BP) and ice streaming (∼13.8 14C kyear BP). (2) The Lough Neagh Basin occupies a similar lowland depression and was the location of an ice sheet centre throughout the last glacial cycle. No bedforms are present beneath or immediately surrounding Lough Neagh. A larger, more continuous meltwater lake existed in the Lough Neagh depression, probably sealed by a region of cold-based ice outside lake margins. Water escaped through regional-scale tunnel valleys, particularly the Poyntzpass channel which was active during the Carlingford ice readvance (Killard Stadial, correlated with Heinrich event 1 at ∼14.5 14C kyear BP). Overall, reconstructed subglacial lake characteristics and drainage mechanisms are related closely to basal ice thermal regime and substrate relief (controlling lake geometry), and provide insight into controls on overall ice sheet dynamics.

The Tolsta Interstadial, Scotland: correlation with D–O cycles GI-8 to GI-5?

Organic sediments, buried by till, at Tolsta Head, in north Lewis in the Scottish Outer Hebridean islands have been re-investigated. Radiocarbon dates of 32–26 kyr BP indicate correlation with the Denekamp Interstadial of The Netherlands and the Sandnes/Ålesund Interstadial of Norway. The organic sediments contain an unusually detailed record of the dominantly open grassland vegetation during the interstadial. Variations in palaeotemperature inferred from the pollen assemblages and sedimentology are provisionally matched with the δ 18O record of the GISP2 ice core and correlation with the Dansgaard–Oeschger cycles GI-8–GI-5 is proposed. The interstadial starts at 38.1k cal yr and 32.8k radiocarbon years. The termination of the interstadial and the build-up of the Late Devensian (Weichselian) ice sheets in Scotland and North West Europe dates from 32k cal yr and 28.7k radiocarbon years. Whether the Tolsta and other contemporaneous episodes are intervals or interstadials is also considered.

Calcareous concretions yield the first U/Th date for the Late Devensian raised marine strata of eastern Scotland

Synopsis This paper presents the first application of U-series disequilibria dating to the Late Devensian raised marine strata of Scotland. The Errol Clay Formation (ECF) is an extensive deposit of laminated glaciomarine silty-clays found offshore from and within the Firths of eastern Scotland. The stratigraphic significance of these deposits lies in their diachronous deposition following the retreat of the Late Devensian ice sheet, and local glaciotectonization following the readvance of the Younger Dryas. In spite of their importance to an understanding of the deglaciation of Scotland, the paucity and poor preservation of biogenic material have hitherto hampered radiocarbon dating of these sediments. A working clay pit at Gallowflat [NO 211 209], in the inner Firth of Tay, provided access to a 5.3 m section of the ECF. A characteristic feature of the lower ECF is that the basal sub-units are rich in calcareous concretions, believed to have formed rapidly at the sediment water interface during glaciomarine sedimentation. Eight individual concretions were hand-picked from the 1.4 m concretion rich sub-unit, 0.5 m above the Devensian till, yielding a U-Th age of 14.2 ±0.9 ka (2σ) using the total-dissolution isochron method. This date accords with the few 14 C dates presently available and offers a new approach to establishing the dynamics of the ECF deglaciation record.

An outline of the lithostratigraphy and depositional history of Quaternary deposits in the Sellafield district, west Cumbria

SUMMARY A lithostratigraphical framework has been erected for Quaternary deposits of the Sellafield district in west Cumbria. It has enabled a sequence of glacial and deglacial events to be established that is one of the most detailed for the last glacial-inter-glacial cycle onshore in Britain and provides new constraints on models of the evolution of the Irish Sea basin during the last 120 000 years. The products of at least one major pre-Devensian (Oxygen Isotope Stage 6 or older) glaciation have been recorded in cored boreholes around Drigg and Nether Wasdale. Ice flowing from Wasdale terminated in proglacial lakes on two occasions during OIS 3 and 4 and an intervening cold-water marine incursion occurred that reached a height of at least 20 m below present sea level, probably during OIS 3. Glaciers emerged again from the Lake District during the build-up of the main Late Devensian ice sheet, which appears to have reached its maximum extent early in OIS 2, when the whole district was glaciated. Scottish ice flowing into the northern Irish Sea basin eventually deflected local ice southwards. The Irish Sea ice stream became dominant again during several readvances that followed a significant deglacial event. The first readvance (Gosforth Oscillation) over-rode most of the coastal plain. The last major one (Scottish Readvance) proglacially tectonized the glacigenic sequence along the coast, forming a push moraine at St Bees. Thick sequences of glaciolacustrine sediment accumulated within the major valleys of the district, both before and after the Gosforth Oscillation, when ice at the coast caused considerable ponding of meltwater inland.

Glacial meltwater erosion of the Mid-Cheshire Ridge: implications for ice dynamics during the Late Devensian glaciation of northwest England

Mapping of glacial meltwater channels along the length of the 25-km Mid-Cheshire Ridge reveals evidence for four distinctive channel morphologies, which are used to establish the pattern of meltwater flow during the Late Devensian glaciation. A key characteristic of all channels is an abrupt change in morphology between inception on the Mid-Cheshire Ridge and the downstream continuation on the surrounding Cheshire Plain, with large reductions in channel cross-sectional area at this point. The interpretation of this evidence is that meltwater flowing off the bedrock ridge was absorbed into a layer of permeable sediment beneath the Late Devensian ice sheet. This permeable sediment is significant because it would have acted as a deforming layer beneath the former ice sheet in this area. Reconstruction of the Late Devensian ice sheet based on information from the meltwater channels and using values of shear stresses typical of ice sheets resting on deformable beds (ca. 20 kPa) suggests an ice surface elevation over the Irish Sea of ca. 700 m. This value is considerably less than previous estimates of the vertical extent of the ice sheet of ca. 1000–1200 m and has important implications for the rapidity and mode of deglaciation during the Late Devensian. Copyright © 1999 John Wiley & Sons, Ltd.

Global meltwater discharge and the deglacial sea‐level record from northwest Scotland

Quantitative models of Earth–ice-sheet–ocean interactions predict that periods of rapid eustatic sea-level rise, indicating enhanced meltwater discharge to the oceans, should be manifest in the relative sea-level histories from sites within the limits of Late Devensian ice sheets. Analysis of a record of relative sea-level change for the last 16000 calendar years from Northwest Scotland constrains the magnitude and timing of two major pulses of meltwater, ca. 14000 and ca. 11300 cal. yr BP, inferred from coral records. The ca. 14000 cal. yr BP event saw a temporary doubling of the flux of meltwater, but existence of the second major event at ca. 11300 cal. yr BP is not supported by the data from northwest Scotland. Copyright © 1999 John Wiley & Sons, Ltd.

Global meltwater discharge and the deglacial sea-level record from northwest Scotland

Quantitative models of Earth–ice-sheet–ocean interactions predict that periods of rapid eustatic sea-level rise, indicating enhanced meltwater discharge to the oceans, should be manifest in the relative sea-level histories from sites within the limits of Late Devensian ice sheets. Analysis of a record of relative sea-level change for the last 16000 calendar years from Northwest Scotland constrains the magnitude and timing of two major pulses of meltwater, ca. 14000 and ca. 11300 cal. yr BP, inferred from coral records. The ca. 14000 cal. yr BP event saw a temporary doubling of the flux of meltwater, but existence of the second major event at ca. 11300 cal. yr BP is not supported by the data from northwest Scotland. Copyright © 1999 John Wiley & Sons, Ltd.

Morphology and palaeoenvironmental interpretation of deformed soft-sediment clasts: examples from within Late Pleistocene glacial outwash, Tempo Valley, Northern Ireland

Abstract Glacial outwash, deposited during deglaciation of the late Devensian ice sheet, is present as a flat-topped valley fill in the Tempo Valley on the southern flanks of the Fintona Hills, Northern Ireland. Sedimentologically, the outwash comprises well-sorted and interbedded rippled to massive sands which record distal deposition within a proglacial water body. Beds of ripple-drift cross-laminated sands contain deformed (folded and contorted) soft-sediment clasts which are composed mainly of silt and clay. The soft-sediment clasts were deformed prior to final deposition because clast a–b planes lie conformable to sand laminae which are undeformed. Morphological characteristics of the soft-sediment clasts, and their facies context, provide evidence for transport mechanisms, depositional environment, and processes of clast deformation. The soft-sediment clasts were transported into a proglacial water body by unidirectional water currents (∼1.5–2.5 m s−1). Sediment transport processes include sediment bypassing within the water column, a low bedload component, and grain flow activity during waning flow stages. The overall morphology of soft-sediment clasts records between 1 and 3 distinct phases of hydroplastic deformation prior to emplacement. The deformation phases are recognised on the basis of morphologically `unrolling' the superimposed folds of the soft-sediment clasts. Deformation structures (i.e. fold style) and direction of the principal stress axis relative to clast axes suggest that clasts were reoriented with respect to water flow direction following each deformation phase. Processes of deformation include folding-over of the clast along its b axis into two or more components, crumpling and abrasion of the outer margins of the b plane, and squashing of the clast c axis (some of which may be post-depositional deformation). The presence of silt- and clay-rich soft-sediment clasts within the outwash succession suggests that they were ripped-up from shallow and irregular pools on the glacier forefield, into which fine sediments accumulated after flood or meltwater events, and transported distally into a proglacial water body. These inferences based on facies evidence and styles of hydroplastic deformation impact on reconstructions of local palaeogeography, and the wider interpretation of similar soft-sediment clasts in the geological record.

Problems of Irish drumlins and Late Devensian ice sheet reconstructions

Reconstructions of the Late Devensian ice sheet in Ireland are strongly biased towards explaining data obtained by the Geological Survey of Ireland during the last century, and observations undertaken by key workers in this century (i.e. J. K. Charlesworth). Modern field observation and analytical techniques, however, show that many of these data, such as the distribution of drumlins, are erroneous. Furthermore, models derived from these data, dealing with the significance and distribution of different bedform types, the location of ice centres and the direction of ice flow lines, can no longer be supported. New observations based on mapping from satellite imagery, and validated by detailed regional-scale field studies, show the complexity of subglacial bedform patterns which record several phases of subglacial activity during the Late Devensian. The presence of ice flow-transverse Rogen moraines and patterns of overprinted bedforms in north central Ireland suggest that subglacial environments and the location of ice centres changed dramatically during the overall glacial cycle.